Neurons

Neurons

• Definition: Neurons are the primary computational and communication units of the nervous system, capable of generating electrical activities and transmitting chemical signals to target cells.

• Glial Cells: Often overlooked, glial cells support neurons and have potential signaling functions beyond mere support.

• Function: Neurons communicate sensory information, drive bodily movements in response to stimuli, and engage in cognitive processes in the brain.

Core Components of Neurons

• Cell Body (Soma)

  • The central part of a neuron containing the nucleus and organelles.

  • Extensions called processes emerge from the cell body, crucial for neuron function.

• Axon

  • A single fiber that protrudes from the cell body and projects to target cells; responsible for propagating nerve impulses.

  • Can branch to communicate with numerous target cells.

• Dendrites

  • Extensions that receive information from other neurons at synapses.

  • Highly branched to allow multiple communication points with the cell body.

  • Information flows through a neuron in one direction: dendrites -> cell body -> axon, creating neuron polarity.

• Axon Hillock

  • Specialized region where the axon emerges; the site of action potential generation in the trigger zone, combining the axon hillock and initial segment.

• Myelin

  • Insulating substance made from glial cells, covering axons and facilitating faster electrical signal transmission via nodes of Ranvier (gaps in myelin).

• Axon Terminal

  • The endpoint of an axon where branches extend into synaptic end bulbs, connecting with target cells at the synapse.

Types of Neurons

• The nervous system has many neurons categorized based on various criteria, primarily the number of processes:

  • Unipolar Neurons

    • Only one process from the cell body (more accurately "pseudo-unipolar" in humans).

    • Primarily sensory neurons receiving information with dendrites at one end.

  • Bipolar Neurons

    • Two processes extending from the opposite ends of the cell body (axon and dendrite).

    • Rare, found in areas such as the olfactory epithelium and retina.

  • Multipolar Neurons

    • Have one axon and two or more dendrites, most common type of neuron, excluding specific sensory neurons.

  • Anaxonic Neurons

    • Very small neurons where processes are indistinguishable under standard magnification; classified as multipolar despite their ambiguous structure.

• Additional Classifications

  • Neurons can also be classified by location, function, neurotransmitter types, or those named after discoverers (e.g., Purkinje cells in the cerebellum).

  • Examples of specific neurons:

    • Pyramidal Cells (multipolar, pyramid-shaped bodies in the cerebral cortex)

    • Purkinje Cells (in the cerebellar cortex and named after discoverer Jan Evangelista Purkinje)

    • Olfactory Cells (in the olfactory epithelium).

Terms to Know

• Olfactory Epithelium: A specialized tissue containing sensory neurons that are responsible for the sense of smell.

• Neurotransmitters: Chemical messengers used to transmit signals across synapses from one neuron to another.

• Ganglia: Clusters of nerve cell bodies located in the peripheral nervous system (PNS).